In many countries where malaria parasites are endemic, the rapid and accurate diagnosis of malaria parasites presents challenges. Of four Plasmodium species, P. falciparum, which can be fatal, must be identified promptly and distinguished from the other Plasmodium species that produce the disease in humans (Moody, A., Clin. Microbiol. Rev. 15 (2002): 66-78).
In addition, most malaria-endemic areas feature infections involving two or more of these species; these mixed infections often go unrecognized or underestimated (Zimmerman, P. A., et al., Trends Parasitol. 20 (2004): 440-447). Failure to detect mixed infection could result in inadequate treatment, and may result in severe disease (Mayxay, M., et al., Trends Parasitol. 20 (2004): 233-240). There is, therefore, an urgent need to develop malaria diagnostic methods that are operable in endemic areas, easy, rapid, highly sensitive and species-specific.
Currently the easy diagnostic method for malaria is microscopic examination of blood smears. Given a high density of parasites, such microscopy has relatively high sensitivity and specificity and provides developmental stage and species determination. However, in endemic areas where parasite density is generally low, this method is labor-intensive, requires well-trained experts, and may result in therapy being delayed.
To improve the speed and precision of malaria diagnosis in regions where standard laboratory diagnosis is not available, researchers have developed rapid diagnostic tests (RDTs) for malaria based on immunoreaction (Moody, A. Clin. Microbiol. Rev. 15(2002): 66-78; Ndao, M., et al., J. Clin. Microbiol. 42 (2004): 2694-2700). However, the sensitivity varies between products (Murray, C. K., et al., Trop. Med. Int. Health. 8 (2003): 876-883), and a species-specific product is available only for P. falciparum. Very long observation times and considerable expertise are required for correct diagnosis by microscopy under several circumstances: when parasitemia is low, during mixed infection, after drug treatment, and during the chronic phase of the infection. Therefore, this situation can lead to false negative results or unreliable species diagnosis (Coleman, R., et al., Thailand. Malar. J. 14 (2006): 121).
Subsequently, molecular-biological methods based on DNA amplification, such as nested PCR and real-time quantitative PCR, were developed for malaria diagnosis. Compared to microscopy, these methods have demonstrated higher sensitivity and greater specificity for mixed infections (Kimura, K., et al., Parasitol. Int. 46 (1997): 91-95; Perandin, F., et al., J. Clin. Microbiol. 42 (2004): 1214-1219; Rougemont, M., et al., J. Clin. Microbiol. 42 (2004): 5636-5643; Singh, B., et al., Am. J. Trop. Med. Hyg. 60 (1999): 687-692; Singh, B., et al., Lancet. 363 (2004): 1017-1024; Snounou, G., et al., Mol. Biochem. Parasitol. 58 (1993): 283-292; Snounou, G., et al., Mol. Biochem. Parasitol. 61 (1993): 315-320). However, the long turnaround time, high cost, and availability only in well-equipped laboratories render this PCR technology inadequate for routine diagnosis in the hospital laboratories and on-site clinics of endemic areas (Hanscheid, T., and Grobusch, M. P., Trends Parasitol. 18 (2002): 395-398).
Regarding malaria detection, Examples 8 and 10 in patent document 1 disclose a method of extracting nucleic acids from blood samples and conducting nested PCR to detect four species of Plasmodium. Example 8 discloses each forward primer and the reverse primer sequences, which are different from the primer sequences of the present invention (patent document 1).
Patent documents 2 and 4 Patent Publication disclose methods for detecting one or multiple species of malaria infection based on a solid phase method or nested PCR, in which one or a plurality of multiple types of primers are used to clinically detect P. falciparum, P. vivax, P. malariae or P. ovale. However, those primers have different primer sequences to those of the present invention.
Patent documents 3 Patent Publication discloses a method for detecting P. falciparum and/or P. vivax, in which P. falciparum and/or P. vivax specific primers are bound to labelor solid supports. However, these specific primer sequences are different from the oligonucleotide sequences of the primer sets of the present invention.
Recently, a novel, easy and highly sensitive technique called loop-mediated isothermal amplification (LAMP) was developed (Notomi, T., et al., Nucleic Acids Res. 28 (2000): e63; WO 2000/28082).
LAMP is a nucleic acid amplification method that relies on auto-cycle strand-displacement DNA synthesis performed by Bst DNA polymerase. The amplified products are stem-loop structures with several repeated sequences of the target, and have multiple loops.
The principal merit of this method is that denaturation of the DNA template is not required, (Nagamine, K., et al., Clin. Chem. 47 (2001): 1742-1743), and thus the LAMP reaction can be conducted under isothermal conditions (ranging from 60 to 65° C.). LAMP requires only one enzyme and four types of primers that recognize six distinct target regions. The method produces a large amount of amplified product, resulting in easier detection, such as detection by visual judgment of the turbidity or fluorescence of the reaction mixture (Mori, Y., et al., Biochem. Biophys. Res. Commun. 289 (2001): 150-154). LAMP in which a fluorescent substance such as fluorescein, fluorescein isothiocyanate (FITC), X-rhodamine (ROX) or the like is used to measure the fluorescence polarization values of the reaction mixture, and LAMP in which SYBR Green 2, a green dye, is used as an intercalator are known (Japanese Unexamined Patent Publication No. 2002-272475, and WO 2002/103053).
Several investigators have reported LAMP methods for the rapid identification of Plasmodium, Trypanosoma, Babesia, Fusarium, Listeria and Legionella, and have recommended the usefulness of LAMP assay (Ikadai, H., et al., J. Clin. Microbiol. 42 (2004): 2465-2469; Kuboki, N., et al., J. Clin. Microbiol. 41 (2003): 5517-5524; Thekisoe, O., et al., Mol. Biochem. Parasitol. 122 (2002): 223-236; Japanese Unexamined Patent Publication No. 2005-245257, Japanese Unexamined Patent Publication No. 2007-61061, Japanese Unexamined Patent Publication No. 2003-219878 and Poon, L., et al., Clin. Chem. 52 (2006): 303-306).
Poon et al., estimated that the cost of running a LAMP assay is about one tenth that of normal PCR for P. falciparum detection (Poon, L., et al., Clin. Chem. 52 (2006): 303-306). The biggest reduction in cost and time came from simple sample preparation without previous DNA extraction (Iwasaki, M., et al., Genome Lett. 2 (2003): 119-126).
For the preparation of samples, simply heating the infected blood at 99° C. for 10 minutes was enough to prepare a DNA template for LAMP (Poon, L., et al., Clin. Chem. 52 (2006): 303-306). However, to date, LAMP for the detection of malaria parasites in clinical diagnosis has been validated only in acute P. falciparum patients (Poon, L., et al., Clin. Chem. 52 (2006): 303-306). Although P. falciparum is the most important cause of severe disease, its geographic distribution overlaps with those of P. vivax, P. malariae and P. ovale infection, and therefore a method allowing the rapid detection and identification of all four species infecting humans would be desirable.
In recent years, in malaria-endemic areas, the development of drug-resistant strains has been a major problem for appropriate malaria treatment. Practicing medical personnel or hospital doctors desire rapid and highly sensitive differentiation methods to obtain information on whether a patient with a fever is infected with a particular malaria parasite or multiple species of malaria parasites, to thereby appropriately treat said malaria patient with a fever.    [Patent document 1] WO2006/88895    [Patent document 2] Japanese Unexamined Patent Publication No. 1994-261758    [Patent document 3] Japanese Unexamined Patent Publication No. 1993-227998    [Patent document 4] Japanese Unexamined Patent Publication No. 2003-250564    [Non-patent document 1] Poon, L., et al., Clin. Chem. 52 (2006): 303-306